1. Field of the Invention
The present invention concerns a power supply circuit to be employed in a power supply device that supplies a constant voltage to electronic equipment, LED lighting, or the like.
2. Description of Related Art
In the past, power supply circuits that rectify and convert alternating current voltage, and output direct current voltage, have been employed as power supply devices for supplying constant electrical current to electronic equipment, LED lighting, and the like. For example, in the case of a power supply device for outputting 100 V alternating current as 100 V direct current, after initial conversion to approximately 380 V direct current by a PFC circuit, the current is stepped down to 100 V by a DC-DC converter(DC-DC converter), and output.
In this sort of power supply device, because the current passes through the PFC circuit and the DC-DC converter, losses occur during passage through the respective circuits, and the total loss is considerable. Moreover, due to the provision of two circuits, i.e., the PFC circuit and the DC-DC converter circuit, the circuit configuration is larger, and production cost is correspondingly higher.
Japanese Patent Application No. 2004-135372 discloses an improved-power factor converter provided with first and second switching elements connected in series between the outputs of a full-wave rectifier circuit; third and fourth switching elements connected in series between output terminals; and a reactor (coil) connected between the node of the first and second switching elements and the node of the third and fourth switching elements, whereby the input voltage is stepped down or stepped up through synchronous on/off control (switching control) of the first to fourth switching elements.
A power supply circuit is disclosed in Japanese Patent Application No. 11-98825 as well. In this power supply circuit, a step-up converter and a step-down converter are arranged in series, and when the input voltage is less than a predetermined voltage, a step-up operation is performed, or when the input voltage is greater than a predetermined voltage, a step-down operation is performed through synchronous control of a switching element included in the step-up converter and a switching element included in the step-down switching converter.
Japanese Patent Application No. 2010-273501 discloses a switching power supply circuit configured by connecting a well-known step-up converter circuit and step-down converter circuit. In the switching power supply circuit, switching control of the switching element included in the step-up converter circuit and of the switching element included in the step-down converter circuit is carried out independently, and a voltage that is higher or lower than the input voltage is output.
Japanese Patent Application No. 2010-273501 also discloses a switching power supply circuit of a configuration provided with three diodes, two coils, two capacitors, and one switching element. By varying the on/off ratio (duty ratio) of the switching element, this switching power supply circuit can be utilized as a step-up converter for stepping up an input voltage, or as a step-down converter for stepping down an input voltage.
Japanese Patent Application No. 2010-268590, proposes a switching power supply circuit in which a coil is employed in common by both a step-up converter circuit and a step-down converter circuit.
Nevertheless, the configuration of the improved-power factor converter disclosed in Japanese Patent Application No. 2004-135372 involves synchronous switching of at least two of the four switching elements, while the power supply circuit disclosed in Japanese Patent Application No. 11-98825 involves synchronous switching of two switching elements during step-down operation, and the efficiency falls because of this switching. Moreover, due to the configuration involving synchronous switching of two switching elements, it is necessary to employ an element for high-speed driving of a control circuit that transmits a control signal to the two switching elements, or to add a processing circuit (element) for signal processing of the control signal, thereby increasing the cost of the power supply circuit.
In the switching power supply circuit shown in Japanese Patent Application No. 2010-273501, coils are necessary for both the step-up converter circuit and the step-down converter circuit, resulting in a greater number of parts. Moreover, because the coils are components that are difficult to make compact and thin, it is difficult to make the switching power supply circuit compact and thin. Further, the use of two coils, which are expensive components, increases the production cost of the switching power supply circuit.
Further, in the switching power supply circuit shown in Japanese Patent Application No. 2010-268590, the connection terminals (in the case of a transistor, the source or emitter) of the first switching element and the second switching element differ in potential (the potential that serves as reference for driving). Therefore, for each of the switching elements, it is necessary to furnish the control circuit with means (for example, a bootstrap circuit) for obtaining a reference potential to drive the first switching element and the second switching element, making the configuration of the switching power supply circuit rather complicated. Moreover, control by means of a control circuit is complicated as well, and therefore it is not easy to reduce production cost.